Regenerating Thumbnails

I am in the process of building a website for the village I live in and specifically for the men who lost their lives in the First World War. On the Life Stories Category page carletonrodearchive.com/category/life-stories/ several of the images have heads of the people cut off in the thumbnails. I have these coming from the featured image where I have tried to crop the thumbnail image. I also installed the regenerate thumbnail plugin to try and help, but nothing seems to change. Can you guide me further or make any suggestions please? Many thanks, Chris